1. Field of the Invention
The present invention relates generally to power supplies. In particular, it the present invention relates to systems and methods for managing the power supply in electronic devices, such as those found in digital cameras.
2. Description of the Related Art
In the recent past, camera technology has greatly advanced. Not only have features, such as flashes, automatic focus devices, and automatic aperture setting devices, been added to traditional cameras, but digital photography has also been introduced. Digital cameras typically use a Charge-Coupled Device (CCD) to record an image. The image is then transferred into a memory device. From the memory device, the image may be output on a monitor or printer.
Each of the camera sub-systems noted above require electrical power. Flash units and automatic focus and aperture motors all draw significant power. For digital cameras, electrical power is even more critical, since the CCD array depends on voltages to capture an image, and volatile memory devices must continually receive electrical power in order to store and retain the captured images. Should electrical power ever be unexpectedly lost, the digital camera would lose any images stored in the volatile memory. For this reason, a reliable power supply is a critical component of modern digital cameras. Generally, modern cameras use lithium ("Li") batteries or nickel-cadmium ("NiCad") batteries in their power supplies, since the voltage they provide remains generally constant as the battery is discharged. Their almost constant voltage output results from their low internal impedance. As a result, the voltage generated by a Li or NiCad battery tends to be very stable regardless of the current load placed on the battery up until the battery is almost completely drained.
Li and NiCad batteries, however, have many limitations, such as being more expensive than more common batteries, such as alkaline batteries, and not being readily available in all stores. For instance, where alkaline batteries are usually available in general merchandise stores, Li and NiCad batteries are typically available only from camera or electronics stores. Furthermore, NiCad batteries discharge even when not in use and may require up to 24 hours or more to re-charge. Such self-discharging creates a risk that any image data stored in a volatile memory may be unexpectedly lost during camera storage.
Many of the inconveniences associated with Li or NiCad batteries are overcome with alkaline batteries. For instance, alkaline batteries are not only much less expensive than Li or NiCad batteries, but also are available in most general merchandise stores. Unfortunately, alkaline batteries have one serious deficiency. When an alkaline battery is relatively new and fully charged, it has a low internal impedance. As the alkaline battery discharges or ages, the internal impedance of the battery increases. Thus, when a high current load is placed on the alkaline battery, the voltage across its terminals dramatically falls as a result of its increased internal impedance. When the current load is decreased, the voltage then returns to a higher and more normal value. The same behavior is observed in both Li and NiCad batteries, but to a much lesser degree than alkaline batteries due to the former's lower internal impedance.
Battery behavior evidencing a falling voltage when the battery is heavily loaded and a rising voltage when the battery load is subsequently reduced is called a "bounce effect." Chemical batteries demonstrate the bounce effect when their voltage dramatically drops upon experiencing a large current drain, such as when a flash unit is charging at full power. After the large current load is removed, the voltage across the chemical battery recovers just as dramatically. The voltage drop created by the bounce effect increases significantly as the chemical battery's internal impedance increases due to age or discharging. Thus, an older and weakened alkaline battery demonstrates a substantial voltage bounce.
The bounce effect's most damaging aspects result when, due to a momentary high current drain event, a battery can no longer produce a large enough voltage to operate the camera, resulting in the complete loss of any image data within the volatile memory. The onset of this problem may occur suddenly, such as when the alkaline battery continues to produce sufficient and decreasing voltages for a period of time until all at once the voltage falls below a minimum level required to operate the camera. This problem is particularly acute when large currents are needed, such as when several of the camera's electronic subsystems are simultaneously energized. For example, if the camera's flash unit is being charged while the focus and aperture motors are adjusting the lens, the substantial current drain on the alkaline battery could cause the camera's voltage to drop below its required minimum.
What is needed is a system and method for automatically compensating for the effects of power supply degradation in order to maximize the power supply's useable life. Such a system and method would optimize camera performance independent of the power supply's operating characteristics.